Electrowetting-on-dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. EWOD is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. These may be termed the electrowetting (EW) drive elements. A droplet 4, consisting of a polar material, is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (e.g. oil) may be used to occupy the volume not occupied by the polar liquid droplet 4. An insulator layer 20, which is frequently, but not necessarily comprised of a dielectric material, is disposed upon the lower substrate 72 separates the conductive electrodes 38A, 38B from a hydrophobic surface 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by angle θ. On the top substrate 36 is another hydrophobic layer 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the hydrophobic layer 26 is a top substrate electrode 28. In operation, voltages, termed the electrowetting (EW) drive voltages, (e.g. VT, V0 and V00) may be externally applied to the different electrodes (e.g. drive element electrodes 28, 38A and 38B, respectively). The hydrophobicity of the hydrophobic surface 16 can be thus be controlled, thus facilitating droplet movement in the lateral plane between the two substrates 72 and 36.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 above further discloses methods for other droplet operations, including the splitting and merging of droplets and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (J. Sterling et al., issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based electronics to control an EWOD array, namely:                Driver circuits can be integrated onto the AM-EWOD array substrate.        TFT-based electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes.                    This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.            U.S. Pat. No. 7,163,612 above does not disclose any circuit embodiments for realizing the TFT backplane of the AM-EWOD.                        
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming and applying an EWOD actuation voltage to the EWOD drive electrode. The voltage write function described includes a memory element of standard means, for example based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element.
US application 2012/0007608 (Hadwen et al., published Jan. 12, 2012) describes how an impedance (capacitance) sensing function can be incorporated into the array element. The impedance sensor may be used for determining the presence and size of liquid droplets present at each electrode in the array.
U.S. Pat. No. 8,173,000 (Hadwen et al; issued May 8, 2012) describes an AM-EWOD device with array element circuit and method for writing an AC actuation voltage to the electrode. This patent further describes methods of driving the device sometimes in an AC and sometimes in a DC mode, so as to be compatible with the operation of integrated sensor functions.
US application 2009/0280251 (DeGuzman et al., published Nov. 12, 2009) describes means for mitigating adsorption of bio-molecules to hydrophobic surfaces in a microfluidic device.